It began in 1923, when South Dakota historian Doane Robinson had the idea to carve larger-than-life figures into the state's Black Hills. Robinson wanted to honor Western heroes — both Native Americans and pioneers — but it was his sculptor, Gutzon Borglum, who suggested the monument go national, spotlighting George Washington and Abraham Lincoln. Robinson appealed to Congress for funds and permission, but only permission was granted. Amid outspoken opposition to the project, planning and fundraising began in 1925. But it wouldn't have gotten very far without President Calvin Coolidge who, in one of his last executive acts, signed a bill approving funding. Borglum broke ground in 1927.

President Calvin Coolidge speaks at the dedication of Mount Rushmore National Memorial on Aug. 10, 1927. | (Photo courtesy of the U.S. National Park Service, photographer Charles D'Emery)

Borglum had originally conceived of the figures with more detail — including clothing and limbs. But the mountain's rough granite forced him to scale back. And while Borglum was the artist with the vision, it was the skilled miners, drillers, and carvers who had to execute. To do so, Borglum first created a model of the sculpture. Then he built a type of measurement machine, inspired by what the Egyptians used to build the pyramids. Situated at the top of the mountain, the machine, along with ratio guidelines (one inch on the model was one foot on the mountain), allowed the workers to transfer the model's mathematical measurements onto the mountain to figure out exactly where to remove rock — and how much.

In the first stage, controlled dynamite detonations blasted the rock into a general shape, exposing the more malleable granite. The dynamite helped to carve 90 percent of the monument. But it was far from done.

Drillers then removed stone with 75-pound jackhammers. The force of these unwieldy tools was so great that, while strapped into a harness, the men had to attach themselves to the mountain with a chain so they weren't propelled off the face with each drill.

Carvers followed, using a technique called "honeycombing" — drilling a series of shallow holes in a closely spaced grid — to remove bits of granite by hand for the more detailed work. Then, using smaller handheld hammers, they defined the delicate features — the facial hair, Roosevelt's glasses, wrinkles around the eyes — and buffed the stone down to the smoothness of a sidewalk.

The workers were paid $8 an hour, which is more than $100 an hour by today's standards. But the work was brutal and dangerous. They had to endure blazing hot summers and bitter cold winters. Just to clock in each morning, they had to climb more than 500 stairs and maneuver over 45 ramps to the top of the mountain. Drillers and carvers strapped into leather harnesses dangled on the side of the mountain, hundreds of feet off the ground. Anytime they needed to change position or come up for the day, a worker hand-cranked the cable. Incredibly, not a single person died in the process, thanks to the intricate, and stringent, infrastructure Borglum designed.

All told, Mount Rushmore took 400 men and women, 14 years, and nearly $1 million to complete. But in a bitter twist of fate, Borglum wouldn't survive to see it finished — he died following complications from surgery, just one season shy of its completion in 1941.

The blasting of Abraham Lincoln's sculpture. | (Photo courtesy of the U.S. National Park Service, photographer Charles D'Emery)

Drillers, suspended in harnesses and fastened with cables to the winches at the top of the mountain, work on George Washington on July, 22, 1929. Gutzon Borglum, sculptor, directs the carving from a projecting ledge at the left. | (AP Photo)

A driller uses a jackhammer drill to honeycomb the granite. | (Photo courtesy of the National Park Service, photographer Lincoln Borglum)

Winch houses were built on top of Mount Rushmore during the construction. Workers in harnesses attached to steel cables would be raised and lowered by the winches while they worked on the carving. | (Photo courtesy of the National Park Service, photographer Lincoln Borglum)

(Photo courtesy of the U.S. National Park Service, photographer Charles D'Emery)

(ClassicStock / Alamy Stock Photo)

A staged photo of Jefferson's eye. | (Photo courtesy of the U.S. National Park Service, photographer Charles D'Emery)

Guzman Borglum and his son, Lincoln, use the tramway. | (Photo courtesy of the U.S. National Park Service, photographer Charles D'Emery)